1. Technical Field of the Invention
This invention relates generally to mixed signal circuitry and more particularly to amplifying circuits.
2. Description of Related Art
Operational amplifiers are used in a wide variety of applications and may be implemented in a variety of ways depending on the application. For instance, a general purpose operational amplifier may be implemented in a single stage consisting of an input transistor pair and a corresponding load. Alternatively, a high end operational amplifier may include multiple stages, where each stage includes a complex implementation to meet impedance requirements, loading requirements, drive requirements, frequency response requirements, rail-to-rail output swing requirements, etc.
When an operational amplifier is implemented using CMOS (complimentary metal oxide semiconductor) technology, it will have some DC (direct current) offset and flicker noise. DC offset is caused by slight mismatches between the components of the operational amplifier, while flicker noise is an inherent low frequency noise produced by CMOS transistors. In general operational amplifier applications, DC offset and flicker noise have negligible effects on the performance of circuitry incorporating the operational amplifier. However, for high end applications, DC offset and flicker noise can adversely affect the desired performance of the operational amplifier and/or of the circuitry in which it is incorporated.
One solution to mitigate the adverse effects of flicker noise and DC offset is to provide a chopping circuit with the operational amplifier. The chopping circuit mixes low frequency energy (e.g., the DC offset, and the flicker noise) to a higher frequency energy that is outside of the frequency band of interest. As such, by moving the DC offset and flicker noise into higher frequency energy, they are out of band and have negligible impact on the performance of the operational amplifier.
In other high end applications, an operational amplifier having an input capable of swinging rail-to-rail (e.g., from a positive rail voltage (e.g., VDD) to a negative rail voltage (e.g., ground or VSS)) may be required. When input rail-to-rail operational amplifiers are implemented using CMOS technology, they experience a signal dependent offset, which can adversely affect the performance of the operational amplifier and/or the circuitry in which it is incorporated. Such a signal dependent offset results due to slight performance differences of p-channel transistors and n-channel transistors within the input rail-to-rail operational amplifier.
Therefore, a need exists for a full swing amplifying circuit that substantially mitigates the adverse affects of flicker noise, DC offset, and/or signal dependent offset.